The present invention is directed to a process for reducing the sulfur content in gasoline to a very low level. Gasoline is generally prepared from a number of blend streams. Typical examples include butanes, light straight run, isomerate, FCC cracked products, hydrocracked naphtha, coker gasoline, alkylate, reformate, added ethers, etc. Of these, gasoline blend stocks from the FCC, the reformer and the alkylation unit account for a major portion of the gasoline pool. FCC gasoline, and if present, coker naphtha and pyrolysis gasoline, generally contribute a substantial portion of the pool sulfur.
Sulfur present in the gasoline pool may be in one of several molecular forms, including thiophenes, mercaptans and disulfides. Typical thiophenes include thiophene (&lt;(CH:CH).sub.2 &gt;S) and its alkylated derivatives, and benzothiophene (alternatively thianaphthene). Typical mercaptans occurring in the sulfur-containing gasoline streams include thiophenol (C.sub.6 H.sub.5 SH), and the alkylthiols from ethanethiol to nonanethiol, with potentially smaller amounts of the higher alkylthiols.
A number of methods have been proposed for removing sulfur from gasoline. In general, hydrotreating is the method of choice, on account of the cost and ease of processing using the catalytic method. However, sulfur removal by hydrotreating is generally accompanied by substantial octane loss, as the olefins in the gasoline are converted to low octane components while the sulfur compounds are being removed. A number of proposals have been made to offset the octane loss associated with gasoline hydrotreating.
According to U.S. Pat. No. 3,957,625, the sulfur impurities tend to concentrate in the heavy fraction of the gasoline and a method for removing the sulfur includes hydrodesulfurization of the heavy fraction of the catalytically cracked gasoline so as to retain the octane contribution from the olefins which are found mainly in the lighter fraction. U.S. Pat. No. 5,290,427 teaches fractionating a sulfur containing gasoline feed, and introducing each fraction in turn into a hydrodesulfurization reactor at spaced locations along the length of the reactor according to boiling point. By this method, low boiling, olefin containing fractions are treated for a relatively shorter time, and higher boiling fractions, with lesser amounts of olefins and higher amounts of sulfur containing molecules, are treated for a relatively longer time. U.S. Pat. No. 5,290,427 further teaches contacting the intermediate product from the hydrodesulfurization reaction zone with an acidic catalyst, reportedly to produce a product having a higher octane number than that of the intermediate product.
In U.S. Pat. No. 4,049,542, Gibson et al. discloses a process in which a copper catalyst is used to desulfurize an olefinic hydrocarbon feed such as catalytically cracked light naphtha. This catalyst is stated to promote desulfurization while retaining the olefins and their contribution to product octane.
While regulations require lower sulfur levels in gasoline, there continues to be a need for higher gasoline octane. These often conflicting objectives serve to highlight the need for new methods for reducing sulfur levels in a gasoline pool while maintaining pool octane.